The present invention relates to a method for welding material including metal by laser beam, and more particularly to a laser welding method for a lap joint, a butt joint, a fillet joint and the like of metal material such as aluminum alloy. In particular, the present invention relates to a laser welding method for seam-welding for these joints.
The aluminum alloy defined by JIS(Japanese Industrial Standard) H4000-H4100 is widely used in an automotive vehicle industry, a light industry and a heavy industry as structural material which has a low specific weight, an anti-corrosive property, an aesthetic appearance and a deep-drawing property but it is known as an alloy on which it is difficult to perform a welding process.
An arc or spot welding such as MIG and TIG and a laser welding using CW type or pseudo CW type laser beam such as CO.sub.2 laser has been carried out for a long time as a welding method for welding such a metal material that would be difficult to weld.
In case of welding by YAG laser, a laser beam is transmitted through an optical fiber and/or a fixed optical system to a condenser lens system to form a circular cross-section beam, which is irradiated to a welding workpiece for carrying out the welding.
However, such a conventional welding method for difficult-to-weld material such as aluminum alloy suffers problems that weld defects such as welding breaks, cracks and blow holes would be likely to occur. In particular with a pulse YAG laser, it is unavoidable to prevent solidification cracks that is one of welding cracks. This makes it difficult to widely use difficult-to-weld material in an industrial field.
It is well known that even in case of the pseudo CW type YAG laser, the higher the weld temperature and the weld speed become, the more the solidification cracks are likely to occur in the difficult-to-weld material by the laser welding.